Generally, networks for transmission of voice, data, and video to and from subscribers fall into two main types: ones in which information is encoded in digital format (i.e., electrical or optical signals that represent ones and zeros) and others where the information is encoded as an analog signal (e.g., a video-transmission network such as is used by cable television operators). Recently, service providers have indicated a desire to converge these services. That is, companies that usually offer digital services (like telephone calls or Internet access) would like to offer analog services (such as television programming). Similarly, companies that have traditionally offered analog services (like cable television) would like to more effectively offer data services (Internet access and the like).
Many of these providers (both analog and digital) have constructed fiber-optic networks for carrying their respective services. These fiber optic networks may carry information in digital or analog format. For example, a laser diode may be rapidly turned “on” and “off” (i.e., brighter and darker) to represent ones and zeros for digital transmission, or a laser diode may transmit signals in analog format by modulating its brightness to correspond to that of a desired analog signal (such as one or more TV channels).
In any such cases, it is generally desirable to transmit data both “upstream” and “downstream” over these fiber networks. For example, when accessing the Internet it is necessary to both transmit requests for web content upstream from the subscriber to the service provider central office, and to receive web content sent downstream from the provider's central office to the subscriber. In the context of cable television, subscribers can send a request, for example, for a certain movie (e.g., pay-per-view), where the request is sent upstream from the subscriber's set-top box to the cable television (CATV) office. The subscriber then receives the requested movie content, which is sent downstream from the CATV office to the subscriber's set-top box, so that it can be displayed on a local television.
In order to facilitate this communication between subscribers and providers, various devices are available. For instance, “duplex” optical devices are designed to transmit and receive data over two separate optical fibers. Such duplex devices have dedicated or unidirectional optical ports, where one of the ports and its respective fiber are for reception (e.g., downstream communications) and the other port and its respective fiber are for transmission (e.g., upstream communications).
Another typical optical device is capable of true bidirectional transmission, where the same fiber carries information in both upstream and downstream directions, with each direction carrying information in a different wavelength. Such a bidirectional device, sometimes referred to as “bidi”, is generally implemented with a combination of optical components, typically including laser diodes as transmitters, photodiodes as receivers, and one or more partially transmissive or reflective elements that ensure the various optical signals are selectively routed to or from their intended transmitters or receivers. Conventional bidi devices include a single optical input/output port and one or more electrical ports for either receiving (e.g., via a photodiode) or transmitting (e.g., via a laser diode). Such conventional bidi designs are typically available in the form of diplexers and triplexers.